Spectrum resources are becoming scarcer with the rapid developments in wireless communications. Research on spectrum for wireless communications has found that a certain frequency (e.g., a frequency allocated to television signals) may be idle in most of the time or not used in most regions and meanwhile there may be a lot of users from multiple systems contending for another frequency. That is, there is imbalance in the use of different spectrum resources. Cognitive Radio (CR) techniques are proposed to address the problem. According to CR techniques, changes in surrounding wireless environment are monitored and an available frequency channel of an authorized system is accessed dynamically and occasionally to perform communications as long as it will interfere with the authorized system.
A CR system can occasionally access an available frequency of an authorized system only on the precondition that traffic of the authorized system will not be impacted by the CR system. Thus, it is required that: (1) the CR system should have the ability of correctly detecting an available frequency channel of the authorized system; (2) the CR system should have the ability of performing spectrum handover, i.e., the CR system should quit using the current working frequency in time after detecting the authorized system appears on the current working frequency.
The spectrum handover is required to minimize its impact on communication signals of the authorized system, i.e., after detecting an authorized user appears, the CR system should quickly quit using the current working frequency (i.e., a working frequency) and select a new frequency (i.e., a target frequency) to establish a cell. Further, CR users should be quickly handed over to the newly established cell during the process to guarantee traffic continuity of the CR users. There are the following conventional mechanisms for spectrum handover.
FIG. 1 is a flowchart illustrating a spectrum handover process according to a first conventional mechanism. A base station of a CR system detects an authorized user appears at the current working frequency (i.e., the working frequency), sends a UE-Specific spectrum handover command to each user equipment (UE) that needs a handover within the cell (which is also referred to as a relevant UE). The UE receives the spectrum handover command and starts the spectrum handover process. After the handover, the base station stops transmission and reception at the working frequency, and recovers the cell at the target frequency.
FIG. 2 is a flowchart illustrating a spectrum handover process according to a second conventional mechanism. A base station of a CR system detects an authorized user appears at the current working frequency (i.e., the working frequency), informs UEs within the cell of an update in system information via a certain manner such as paging or the like. A UE receives the paging information, obtains a system information update notification. The base station of the CR system broadcasts updated system information (i.e., system broadcast information including a cell-common spectrum handover command). The UE receives the spectrum handover command, and starts the spectrum handover process. Then the base station stops transmission and reception at the working frequency, and recovers the cell at the target frequency.